In recent years, from the viewpoint of environmental concerns, fuel consumption, and the exhaust gas of automobiles, development of fuel injectors for automobiles using a laminated piezoelectric element is progressing.
The laminated piezoelectric element usually contains a ceramic laminate that is formed by having alternately laminated piezoelectric layers made of a piezoelectric material and internal electrode layers having conductivity. A voltage is applied to between internal electrodes, thereby generating a displacement in the piezoelectric layers for driving.
Laminated piezoelectric elements can have various shapes. For example, there is a hollow laminated piezoelectric element that has a ceramic laminate having a through hole provided in a lamination direction.
Examples of the hollow laminated piezoelectric element that is used as a piezoelectric actuator for a fuel injector of an automobile are described in JP-A-10-9084 and JP-A-2002-252381.
However, the hollow laminated piezoelectric element has a through hole provided in the ceramic laminate. Therefore, it is difficult to insulate the internal peripheral surface of the ceramic laminate, and sufficient reliability cannot be obtained.
In order to solve the above problem, a method of insulating the internal peripheral surface of the ceramic laminate of the hollow laminated piezoelectric element by forming an insulator on the internal peripheral surface using electrophoresis is proposed in JP-A-7-176802 and JP-A-9-329068.
However, according to the above insulation method, the adhesive strength of the insulator is not sufficient, and stress generated by piezoelectric displacement is repetitively applied, thereby causing peeling-off of the insulator. Consequently, moisture enters the internal peripheral surface, and causes an insulation failure.
In addition to the above method, there is also a method of securing insulation on the internal peripheral surface of the ceramic laminate by employing a partial electrode structure in the hollow laminated piezoelectric element.
The laminated piezoelectric element includes a whole surface electrode structure type in which the ends of the internal electrode layers are exposed on all side surfaces of the ceramic laminate, and a partial electrode structure type (or an internally terminated structure) in which a portion of the ends of the internal electrode layers is terminated within the body of the ceramic laminate. According to the whole surface electrode structure type, the ends of the internal electrode layers are exposed on all side surfaces of the ceramic laminate. Therefore, there are many non-insulation parts, and it becomes difficult to insulate these parts. On the other hand, according to the partial electrode structure type, the ends of the internal electrode layers are terminated within the body of the ceramic laminate, and insulation can be secured.
However, in the case of the partial electrode structure type, stress generated due to piezoelectric displacement is concentrated at the ends of the internal electrode layers, and this has a risk of the occurrence of cracks. This inconvenience becomes extreme when a high output is produced. Therefore, it is difficult to obtain sufficient durability and reliability of the piezoelectric actuator for a long-term use as well as to obtain a high output.
As explained above, according to the conventional hollow laminated piezoelectric element, it is not easy to secure insulation, obtain sufficient durability and reliability, and obtain a high output.